Since the discovery of expansin by Cosgrove and his colleagues (McQueen-Mason et al., 1992, Plant Cell 4, 1425-1433), intensive studies have been conducted thereon. In early studies, expansins were known as cell-wall-loosening enzymes that mediate, at least in part, pH-dependent extension of the plant cell wall and the growth of the cell (Cosgrove, 2000, Nature 407, 321-326). Since then, expansins were found to be in either α- or β-form (Shcherban et al., 1995, PNAS 92, 9245-9249).
More recently, expansins have been found to be involved in regulating, besides cell expansion, a variety of other plant processes, including morphogenesis (Ruan et al., 2001, Plant Cell 13, 47-60), softening of fruits (Rose et al., 2000, Plant Physiology 123, 1583-1592; Civello et al., 1999, Plant Physiology 121, 1273-1280), growth of the pollen tube (Cosgrove et al., 1997, PNAS 94, 6559-6564), elongation of graviresponding roots (Zhang and Hasenstein, 2000, Plant Cell Physiology 41, 1305-1312), and elongation of root cells (Lee et al., 2003, Plant Physiology 131, 985-997) (for review, Lee et al., 2001, Cur. Opin. Plant Biol. 4, 527-532).
Further, the expression pattern of expansins in flooded rice and tomatoes has been well studied. It has been found that expansins are expressed in the shoot apical meristem of tomato for incipient leaf primordium initiation (Reinhardt et al., 1998, Plant Cell 10, 1427-1437). An expansin gene (Exp1) was cloned and found through transformants therewith to play an important role in the growth and ripening of tomato fruits in (Brummell et al., 1999, Plant Cell, 11: 2203-2216). Expansin mRNA was accumulated just before the rate of growth or the loosening degree of the cell wall started to increase, suggesting that the expression of expansin genes is correlated with cell elongation (Cho and Kende, 1997a, Plant Cell 9, 1661-1671; 1997b, Plant Physiology 113, 1137-1143; 1998, Plant Journal 15, 805-812). Transgenic rice plants in which expansins are overexpressed were observed to further increase the length of cotyledons by 31-97% compared with the wild type (Choi et al., 2003 Plant Cell, 15: 1386-1398). However, the transgenic rice plants are unable to bear seeds due to male sterility.
On the other hand, sweetpotato storage roots are a good energy source for people because they contain a lot of starch and various kinds of inorganic nutrients, and are high value-added crops having beneficial health effects owing to their high content of fiber, which is a material useful in the body.
Recently, as bio-ethanol obtained upon the fermentation of plants comes into the spotlight as an alternative energy source, sweetpotato storage roots are also being considered as a useful alternative energy crop.
It is very important to increase the productivity per unit area under cultivation of alternative energy crops in view of enhancing the price competitiveness of alternative energy.
Further, there has been little research on the molecular mechanisms of storage root production because the material of the storage root is not suitable for molecular study, for the following reasons: (1) it is not easy to extract DNA and RNA because of the large amounts of polysaccharide; and (2) it is not easy to monitor storage root growth because the storage root grows in the ground, and thus studies into molecular breeding to regulate the development of sweetpotato storage root have been limited.
Therefore, there has been a need for molecular breeding to regulate the development of sweetpotato storage root and transgenic sweetpotato that can remarkably increase production using the molecular breeding.